1. Field of the Invention
Oxygen deficient manganese compounds of the formula CaMnO.sub.2.5 and Ca.sub.2 MnO.sub.3.5 and having a structure related to perovskites. More particularly, oxygen deficient manganese compounds having a perovskite-type structure and of the formula CaMnO.sub.2.5 and Ca.sub.2 MnO.sub.3.5 have been prepared by the relatively low temperature reduction of precursors of the formula CaMnO.sub.3 and Ca.sub.2 MnO.sub.4, respectively.
2. Background of the Disclosure
The term perovskite refers to a large group of compounds related by composition and structure to the mineral calcium titanate (CaTiO.sub.3). A survey of these materials has been compiled by Goodenough and Longo in an article titled "Crystallographic and Magnetic Properties of Perovskite and Perovskite Related Compounds", published in the Landolt-Bornstein Series Group III, v. 4a (1970). There are a large number of stoichiometric perovskite compounds having the general formula ABX.sub.3 wherein A is a large Group IA or IIA cation such as Ba, Sr, Ca or Cs capable of close packing with anions X from Group VI or VIIA such as O, S, Cl and Br in the ratio of one cation to three anions and where B is a smaller cation including transition metals such as Ti, V, Mn and Co capable of fitting the anion octahedra formed by the AX.sub.3 close packed layers. In addition to these, however, some have been found to exhibit nonstoichiometry on one or more sublattices.
In general, the synthesis of nonstoichiometric compounds from, and having a crystal structure closely related to, a stoichiometric precursor depends on the structure of the precursor starting material. This is because the structure has a large influence on reactivity of the precursor. Examples of such nonstoichiometric compounds include intercalation compounds of graphite and other layered compounds, cation exchangeable layer silicates and the oxide shear phases of Group VB and VIB transition metals. Synthesis of materials of this kind, which depend on existing structural elements in the starting material, often result in metastable compounds and are usually found only at low temperatures. However, anion-deficient ABO.sub.3-X perovskites have traditionally been prepared by reducing the oxidized ABO.sub.3 phase at high (i.e., .gtoreq.1,000.degree. C.) temperatures. Thus, compounds which are metastable or stable only at lower temperatures are lost.
A considerable amount of work has been done in synthesizing and studying various compounds containing calcium, manganese and oxygen. Among these are U.S. Pat. No. 4,060,500 which discloses a process for preparing high surface area mixed metal oxides by decomposing solid solutions of carbonates possessing the calcite structure. This patent discloses, for example, that CaMnO.sub.3 with a surface area of 11 m.sup.2 /g useful as a battery cathode can be formed by decomposing a CaMn(CO.sub.3).sub.2 precursor. U.S. Pat. No. 4,049,790 discloses the preparation of a low temperature, layered manganese compound of the formula Ca.sub.2 Mn.sub.3 O.sub.8 by calcining certain solid solutions of mixed calcium and manganese carbonates having a calcite structure. U.S. Pat. No. 4,101,716 discloses the preparation of other high surface area, mixed metal oxides of manganese and calcium which are useful in electrochemical processes. Finally, in an article titled "Phase Relations in the Ca-Mn-O System", in Mat. Res. Bull. v. 13p. 1359-1369 (1978), Longo and Horowitz published an article disclosing what was then the state of the art for both actual and proposed phase diagrams for the Ca-Mn-O system. These phase diagrams represent all that is known or was known about this system until the instant invention.